JP2013161771A - Lithium ion secondary battery and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery capable of improving battery capacity without lowering reliability by an electrode film with a protective layer having high adhesion to the electrode film, and a method for producing the lithium ion secondary battery For the purpose.
[Solution]
The present invention provides an electrode film that includes a binder that is formed on the surface of the current collector foil and adheres to the current collector foil, and a protection that includes the binder that is formed on the surface of the electrode film and adheres to the electrode film and insulating particles. The lithium ion secondary battery is characterized in that the concentration of the binder on the electrode film side in the protective film is higher than the concentration of the binder on the opposite side to the electrode film.
[Selection] Figure 3

Description

  The present invention relates to a lithium ion secondary battery and a method for manufacturing the same.

  As background art in this technical field, there is JP-A-7-220759 (Patent Document 1). This publication states that “the protective layer formed on the surface of the active material layer can prevent the active material from dropping and reattaching after the active material layer is formed and before the electrode is housed in the battery can. The internal short circuit of the battery induced by the active material reattached to the electrode surface can be prevented, and a highly reliable and safe non-aqueous electrolyte secondary battery can be obtained.

Japanese Patent Laid-Open No. 7-220759

  With the development of portable electronic devices, small secondary batteries that can be repeatedly charged are used as power supply sources for these portable electronic devices. Among these, lithium ion secondary batteries that have high energy density, long cycle life, low self-discharge property, and high operating voltage are attracting attention. Lithium ion secondary batteries have the advantages described above, and are therefore widely used in portable electronic devices such as digital cameras, notebook personal computers, and mobile phones.

  Furthermore, in recent years, research and development of large-sized lithium ion secondary batteries capable of realizing high capacity, high output, and high energy density as electric vehicle batteries and power storage batteries have been promoted. In particular, in the automobile industry, in order to cope with environmental problems, development of an electric vehicle that uses a motor as a power source and a hybrid vehicle that uses both an engine (internal combustion engine) and a motor as a power source are in progress. . Lithium ion secondary batteries have attracted attention as power sources for such electric vehicles and hybrid vehicles.

  In the manufacturing process of such a lithium ion secondary battery, a step of winding a positive electrode plate and a negative electrode plate made of metal foil, a step of welding and fixing an electrode winding body to an outer container, and an electrolyte solution in an outer container There is a process of injecting into the inside. For example, in the step of winding a positive electrode plate or a negative electrode plate, metal fine powder may be generated when the positive electrode plate or the negative electrode plate, which is a metal foil, is wound. Moreover, in the process of welding and fixing the electrode winding body to the outer container, there is a possibility that metallic foreign matter may be scattered during welding. Furthermore, in the step of injecting the electrolytic solution into the exterior container, there is a possibility that a metal foreign matter mixed in the electrolytic solution may enter the electrode winding body. From the above, in the manufacturing process of the lithium ion secondary battery, there is a potential (potential) that a metal foreign substance is mixed inside the electrode winding body.

  Since the porous and insulating protective film described in Patent Document 1 is provided on the surface of the electrode film, it is possible to prevent the adhesion of the metal foreign object to the electrode surface. The internal short circuit due to can be suppressed. As a method for forming such a protective film, a method of applying a slurry made of insulating particles and a binder component on the surface of the electrode film and drying it can be employed. However, in the conventional manufacturing method, since the binder distribution in the protective film is generated, the adhesion with the electrode film is lowered, and the protective film is easily dropped during the transportation of the electrode plate or the manufacturing process of the electrode winding body. In order to improve the reliability, it is necessary to increase the binder content. When the content of the binder is excessively increased, the battery capacity is lowered, and it is difficult to achieve both improvement in reliability and improvement in battery capacity.

  In view of the above problems, the present invention provides a lithium ion secondary battery capable of improving battery capacity without reducing reliability by using an electrode film with a protective layer having high adhesion to the electrode film, and a method for manufacturing the lithium ion secondary battery The purpose is to provide.

  In order to solve the above-mentioned problems, the present invention provides an electrode film that is formed on the surface of a current collector foil and includes a binder that adheres to the current collector foil, and a binder that is formed on the surface of the electrode film and adheres to the electrode film. And a protective film containing insulating particles, wherein the concentration of the binder on the electrode film side in the protective film is higher than the concentration of the binder on the opposite side of the electrode film I will provide a.

  The present invention also includes a first step of applying a liquid protective film slurry containing a binder that adheres to the electrode film on the electrode film formed on the surface of the current collector foil; and a solvent containing a solidified liquid is added to the protective film. A lithium ion secondary battery comprising: a second step of bringing the protective film slurry into contact with a slurry and solidifying the protective film slurry; and a third step of removing a liquid component from the solidified protective film slurry and drying the solid film. A manufacturing method is provided.

  According to the present invention, there is provided a lithium ion secondary battery capable of improving battery capacity without reducing reliability by an electrode film with a protective layer having high adhesion to the electrode film, and a method for manufacturing the lithium ion secondary battery. Can do.

It is a figure showing the protective film formation process in the conventional method. It is a figure which shows the concentration to the protective film surface side of the binder after the conventional protective film drying. It is a figure showing the formation process of the protective film in this invention embodiment. It is a figure which shows binder distribution of a protective film at the time of forming a protective film in the shape of an electrode film by sequential lamination | stacking, and an electrode film. It is a figure which shows the binder distribution of a protective film and electrode film at the time of forming a protective film in electrode film shape by collective formation. It is a figure which shows binder distribution of the protective film and electrode film which formed the protective film from which binder concentration differs.

  Hereinafter, embodiments of the present invention will be described in detail.

  The structure of the lithium ion secondary battery in this embodiment is demonstrated using FIGS. 1-3.

  The lithium ion secondary battery in the present embodiment is wound or stacked via a separator that prevents contact between the positive electrode plate and the negative electrode plate. After the wound body or the laminated body is stored in the battery outer container, the electrolytic solution is injected into the outer container. As shown in FIG. 3, the positive electrode plate and the negative electrode plate have a structure in which an electrode film 9 is formed on a current collector foil 6 that is a metal foil as a current collector, and a protective film 8 is formed on the surface of the electrode film 9. ing. The electrode film 9 is an electrode film slurry containing an active material 3 capable of releasing and occluding lithium ions by charge and discharge, a binder 4 for bonding the current collector foil 6 and the electrode film 9, and a solvent 2 in which the conductive material 5 is dissolved. It is formed by solidifying and drying. The protective film 8 is formed by solidifying and drying the protective film slurry containing the solvent 2 in which the insulating particles 1, the electrode 4 and the binder 4 that bonds the protective film 8 are dissolved.

  The outline of the manufacturing process of the lithium ion secondary battery in the present embodiment will be described. First, the electrode film slurry containing the active material 3, the binder 4, the conductive material 4, and the first solvent is applied to the current collector foil 6, and the second solvent containing the solidified liquid 11 is brought into contact with the first electrode film slurry. The electrode film 9 is formed by solidifying, removing the liquid component, and drying. Moreover, the protective film slurry containing the insulating particles 1, the binder 4, and the first solvent is applied to the electrode film 9, and the second solvent containing the solidified liquid 11 is brought into contact with the protective film slurry to solidify the liquid component. The protective film 8 is formed by removing and drying. By these steps, each of the positive electrode plate and the negative electrode plate is formed. Subsequently, a separator made of a porous insulating material that allows lithium ions to pass through is sandwiched between the positive electrode plate and the negative electrode plate while preventing contact between the positive electrode plate and the negative electrode plate, and the positive electrode plate, the separator, and the negative electrode plate are wound. Thereby, the electrode winding body which consists of the wound positive electrode plate, separator, and negative electrode plate can be formed. Next, after inserting and fixing this electrode winding body in the exterior container of a battery, electrolyte solution is inject | poured inside the exterior container. And a lithium ion secondary battery can be manufactured by cap-sealing an exterior container.

  A conventional method for forming the protective film 8 will be described with reference to FIGS. Conventionally, a liquid protective film slurry is applied to the surface of the electrode film 9 formed on the surface of the current collector foil 6 and introduced into a drying chamber as it is to be dried.

  That is, as shown in FIG. 1, in the drying process of the applied protective film 8, the solvent 2 in the applied protective film 8 evaporates and dries, but the protective film 8 is applied to the surface of the electrode film 9. The solvent 2 evaporates from the surface of the protective film 8 (opposite side of the electrode film 9). As the drying proceeds, the solvent 2 on the electrode film 9 side moves to the surface of the protective film 8 (opposite side of the electrode film 9) and evaporates from the surface of the protective film 8 (opposite side of the electrode film 9).

  As the solvent 2 evaporates, the binder 4 dissolved in the solvent 2 moves together with the solvent 2 toward the surface of the protective film 8 in the direction of the arrow in FIG. The moved solvent 2 evaporates as a solvent 7 evaporated from the surface, but the dissolved binder 4 is deposited and remains with the evaporation of the solvent 2. By this mechanism, the binder 4 is concentrated on the surface side of the protective film 8 (opposite side of the electrode film 9), as shown in FIG.

  A method of forming the protective film 8 in this embodiment will be described with reference to FIG. In this embodiment, a step of solidifying the applied protective film slurry is added, and the solidified protective film 8 is dried. By using this method, the decrease in the adhesion of the protective film 8 is eliminated. The decrease in the adhesion of the protective film 8 causes the binder distribution in the protective film 8 to become non-uniform during the drying process, and the binder concentration on the electrode film 9 side becomes lower than the binder concentration on the surface side (opposite side of the electrode film 9). This is what happens. In this embodiment, by adding a step of solidifying the applied protective film slurry, the binder concentration on the electrode film 9 side can be increased also on the surface side (opposite side of the electrode film 9). Furthermore, the difference between the binder concentration on the electrode film 9 side and the binder concentration on the surface side (opposite side of the electrode film 9) can be 50% or less of the binder concentration on the surface side (opposite side of the electrode film 9), It is possible to prevent the binder distribution in the protective film 8 from becoming non-uniform.

  The same applies to the electrode film 9, and in this embodiment, a step of solidifying the applied electrode film slurry is added, and the solidified electrode film 9 is dried. By using this method, the decrease in the adhesion of the electrode film 9 is eliminated. The decrease in the adhesion between the current collector foil 6 and the electrode film 9 causes the binder distribution in the electrode film 9 to be non-uniform during the drying process, and the binder concentration on the current collector foil 6 side is lower than the binder concentration on the protective film 8 side. This is what happens. In this embodiment, by adding a step of solidifying the applied electrode film slurry, the binder concentration on the current collector foil 6 side can be made higher than the binder concentration on the protective film 8 side. Furthermore, the difference between the binder concentration on the current collector foil 6 side and the binder concentration on the protective film 8 side can be 50% or less of the binder concentration on the protective film 8 side, and the binder distribution in the electrode film 8 becomes non-uniform. Can be suppressed.

  In the method of solidifying the binder 4 of the present embodiment, the second solvent containing the solidified liquid 11 of the present embodiment is applied to the coating protective film 8 held on the surface of the electrode film 9 containing the first solvent of the present embodiment. What is necessary is just to deposit the binder 4 by making it contact. Accordingly, a method of passing the coating film held on the surface of the electrode film 9 through the liquid tank storing the second solvent, a method of spraying the second solvent on the coating film held on the surface of the electrode film 9, Although the system etc. which supply the solvent of 2 while flowing down are included, it is not limited to these.

  The same applies to the method of solidifying the binder 4 in the electrode film 9 by bringing the second solvent containing the solidified liquid 11 of the present embodiment into contact with the surface of the electrode film 9 containing the first solvent of the present embodiment. What is necessary is just to precipitate the binder 4. Accordingly, a method of passing the coating film held on the surface of the electrode film 9 through the liquid tank storing the second solvent, a method of spraying the second solvent on the coating film held on the surface of the electrode film 9, Although the system etc. which supply the solvent of 2 while flowing down are included, it is not limited to these.

  In this embodiment, since the solidification is caused by precipitation of the binder 4 dissolved in the first solvent, the binder distribution is basically the same as that of the protective film slurry for coating when the solidification is instantaneous, that is, the binder 4. Is uniformly distributed in the protective film 8. On the other hand, in the actual solidification of the binder 4, the solidified liquid 11 penetrates from the surface of the protective film 8 as shown by the arrows in FIG. 3, and the binder 4 dissolved in the first solvent is precipitated by the solidified liquid 11. That is, the precipitation of the binder 4 proceeds from the surface to the electrode film 9 side, and the binder concentration on the electrode film 9 side increases. In contrast to the binder 4 on the surface side of the protective film 8 (on the side opposite to the electrode film 9), the binder distribution increases on the electrode film 9 side. A good protective film 8 is obtained.

  The same applies to the electrode film 9. The solidified liquid 11 penetrates from the surface of the electrode film 9, and the binder 4 dissolved in the first solvent is precipitated by the solidified liquid 11. That is, the precipitation of the binder 4 proceeds from the surface to the current collector foil 6 side, and the binder concentration on the current collector foil 6 side increases. Contrary to the large amount of binder 4 on the surface side (protective film 8 side) of the conventional electrode screen 9, this binder distribution increases the amount of binder 4 on the current collector foil 6 side, so that the adhesion to the current collector foil 6 is improved. A good electrode film 9 is obtained.

  The lower limit of the contact time required for solidification generally requires time for the first solvent and the second solvent to interdiffuse and replace in the coating film, but the thickness of the protective film 8 is 1 mm. If it is below, the contact time is preferably 1 to 100 seconds, more preferably 2 to 50 seconds, and even more preferably 5 to 20 seconds.

  The movement of the binder 4 accompanying the drying after the binder 4 is solidified in the present embodiment does not occur. Therefore, the method of drying the protective film 8 is not limited to general hot air drying. A heating method that irradiates electromagnetic waves such as infrared rays, far-infrared rays, or visible light may be used, or a dielectric heating method that uses a high-frequency electric field, or an induction heating method that uses a change in magnetic flux may be used. . Furthermore, a contact heating method using a heating roll or a hot plate incorporating a heater can also be used.

As the insulating particles 1 constituting the protective film 8 used in the present embodiment, various inorganic particles and / or resin particles exhibiting insulating properties (non-conductive properties) can be used. From the viewpoint of durability and reliability, it is preferable to use inorganic particles. For example, the inorganic particles may be metal element or non-metal element oxides, carbides, silicides, and nitrides. From the viewpoint of chemical stability and material cost, it is preferable to use oxide particles such as alumina (Al ₂ O ₃ ) and silica (SiO ₂ ).

  The average particle diameter of the insulating particles 1 to be used is related to the thickness of the protective film 8 formed on the surface of the electrode film 9, but is preferably about 0.1 to 10 μm, and more preferably about 0.3 to 5 μm. It becomes. Moreover, the insulating particle | grains 1 which mixed the particle | grains of several average particle diameter can also be used.

  As the binder 4 constituting the protective film 8 used in the present embodiment, for example, a polyvinylidene fluoride polymer (vinylidene fluoride which is a main component monomer) is used as a polymer material having the property of binding the insulating particles 1 described above. (Polymers of fluorine-containing monomers containing 80% by mass or more), rubber-based polymers, and the like are preferably used. Two or more of the above polymers may be used in combination.

  Examples of the fluorine-containing monomer group for synthesizing the polyvinylidene fluoride-based polymer include vinylidene fluoride; a mixture of vinylidene fluoride and another monomer, and a monomer mixture containing 80% by mass or more of vinylidene fluoride; Can be mentioned.

  Examples of other monomers include vinyl fluoride, trifluoroethylene, trifluorochloroethylene, tetrafluoroethylene, hexafluoropropylene, and fluoroalkyl vinyl ether.

  Examples of the rubber-based polymer include styrene butadiene rubber (SBR), ethylene propylene diene rubber, and fluorine rubber.

  Moreover, the binder 4 of this embodiment may be added separately from the component which has the performance as the solidification liquid 11, and the binder 4 itself may have a function as the solidification liquid 11. When the binder 4 is added separately from the component having the performance as the solidifying liquid 11, the above polymer material having the property of binding the insulating particles 1 is preferably used as the binder 4, but the solution dissolved in the solvent 2 is not necessarily used. However, it may be in the form of an emulsion in which a polymer material is dispersed in a liquid.

  It is preferable that at least one component of the binder 4 used for forming the electrode film 9 and the binder 4 used for forming the protective film 8 are the same. Furthermore, the binder 4 used for both the electrode film 9 and the protective film 8 is more preferably a polymer material having the properties of the solidified liquid 11 in this embodiment. When each binder 4 contains the same component, an integrated film in which the electrode film 9 and the protective film 8 are more closely attached can be obtained, and the durability of the protective film 8 can be improved.

  The content of the binder 4 in the protective film 8 is 0.1% by mass or more, more preferably 0.5% by mass or more and 20% by mass or less, more preferably 10%, based on the protective film 8 after drying. It is desirable that it is less than mass%. If the content of the binder 4 is too small, not only is the solidification in the solidification step of this embodiment insufficient, but the mechanical strength of the mixture layer after drying is insufficient, and the protective film 8 is peeled off from the electrode film 9. May fall. Moreover, when there is too much content of the binder 4, the pore formed with the insulating particle 1 will be obstruct | occluded by the binder 4, and battery performance will deteriorate by the porosity decreasing (battery capacity fall etc.). There is a risk of causing.

  It is important that the solvent 2 of this embodiment is used by appropriately selecting the first solvent and the second solvent. The solvent 2 should be selected from the solubility of the solidified liquid 11 of the present embodiment or the component of the binder 4 that also serves as the solidified liquid 11 and the mutual solubility of the solvent. As the first solvent, N-methylpyrrolidone, dimethylsulfoxide An aprotic polar solvent represented by propylene carbonate, dimethylformamide, γ-butyrolactone or a mixture thereof can be selected. As the second solvent, a protic solvent typified by water, ethanol, isopropyl alcohol, acetic acid or the like, or a mixed solution thereof can be selected, but is not limited to the examples given here. In some cases, aliphatic saturated hydrocarbons, aliphatic amines, esters, ethers, various halogen-based solvents, and the like can be selected as the second solvent. Further, in some cases, it is possible to select to exchange the first solvent and the second solvent. The selection of the solvent 2 in this embodiment depends on the selection of the solidifying component used for the protective film 8 and the combination of the two types of solvents 2 that match the selection.

  The solvent used for the electrode film slurry and the protective film slurry may be the same or different. In the technology disclosed herein, the same solvent as the electrode film 9 slurry can be preferably used as the solvent of the protective film slurry (first solvent in the present embodiment).

  Further, it is necessary to select and use a solvent that does not melt or swell the material of the binder 4 contained in the electrode film 9 as the second solvent. In the technique disclosed herein, the protective film 8 is formed by bringing the second solvent into contact with the coating protective film 8 held on the surface of the electrode film 9 containing the first solvent, and precipitating and solidifying the binder 4. Since the second solvent also contacts the electrode film 9, an event that affects the state of the electrode film 9 can be avoided by selecting a solvent that does not melt or swell the material of the binder 4 contained in the electrode film 9. .

  As a method for applying the electrode film slurry and the protective film slurry of the present embodiment, various application methods such as an extrusion coater, a reverse roller, a doctor blade, an applicator and the like can be adopted.

  Among the active materials 3, as the positive electrode active material used for the positive electrode, lithium cobaltate, a lithium-containing composite oxide having a spinel structure containing manganese, a composite oxide containing nickel, cobalt, manganese, or olivine An olivine type compound represented by type iron phosphate is used, but is not limited thereto. Since the lithium-containing composite oxide having a spinel structure containing manganese is excellent in thermal stability, for example, a highly safe battery can be configured. As the positive electrode active material, only a lithium-containing composite oxide having a spinel structure containing manganese may be used, but another positive electrode active material may be used in combination. Examples of such other positive electrode active materials include olivine type compounds represented by Li1 + xMO2 (−0.1 <x <0.1, M: Co, Ni, Mn, Al, Mg, Zr, Ti, etc.) Is mentioned. Specific examples of the lithium-containing transition metal oxide having a layered structure include LiCoO2 and LiNi1-xCox-yAlyO2 (0.1 ≦ x ≦ 0.3, 0.01 ≦ y ≦ 0.2), and at least Co. An oxide containing Ni and Mn (LiMn1 / 3Ni1 / 3Co1 / 3O2, LiMn5 / 12Ni5 / 12Co1 / 6O2, LiNi3 / 5Mn1 / 5Co1 / 5O2, etc.) can be used.

  Among the active materials 3, examples of the negative electrode active material used for the negative electrode include graphite materials such as natural graphite (flaky graphite), artificial graphite, and expanded graphite; graphitizable properties such as coke obtained by firing pitch. Carbonaceous materials: Carbon materials such as non-graphitizable carbonaceous materials such as amorphous carbon obtained by low-temperature firing of furfuryl alcohol resin (PFA), polyparaphenylene (PPP), and phenol resin. In addition to the carbon material, lithium or a lithium-containing compound can also be used as the negative electrode active material. Examples of the lithium-containing compound include lithium alloys such as Li—Al, and alloys containing elements that can be alloyed with lithium such as Si and Sn. Furthermore, oxide-based materials such as Sn oxide and Si oxide can also be used.

  The conductive material 5 is usually used as an electron conduction aid to be contained in the positive electrode film. For example, carbon materials such as carbon black, acetylene black, ketjen black, graphite, carbon fiber, and carbon nanotube are preferable. Among the above carbon materials, acetylene black or ketjen black is particularly preferable from the viewpoint of the amount of addition and conductivity, and the productivity of the positive electrode mixture slurry for coating. Such a conductive material 5 can be contained in the negative electrode film, and may be preferable.

  The current collector foil 6 used in the present embodiment is representatively shown, and is not limited to a sheet-like foil. Examples of the substrate include pure aluminum, copper, stainless steel, titanium, and the like. A metal, an alloy conductive material, or a net, a punched metal, a foam metal, a foil processed into a plate shape, or the like is used. As the thickness of the conductive substrate, for example, 5 to 30 μm, more preferably 8 to 16 μm is selected.

  The lithium ion secondary battery that can be provided by the present embodiment can be manufactured in the same manner as a conventional secondary battery except that it includes a positive electrode and a negative electrode manufactured by the above-described method. There is no particular limitation on the structure and size of the battery container or the structure of the electrode body having positive and negative electrodes as main components.

  The manufacturing method of the electrode film 9 with the protective film 8 in which the binder 4 according to the present embodiment is uniformly distributed and has high adhesion has been described above. Hereinafter, preferred examples of the present embodiment will be described based on experimental results. explain. In addition, the density | concentration of the binder 4 is shown by the mass%.

  Here, the case where the protective film 8 is formed on the positive electrode film in the electrode film 9 by sequential application will be described.

  First, formation of the positive electrode film will be described. Among the electrode film slurries, the positive electrode film slurry was prepared by the following method. As the active material 3, a lithium transition metal composite oxide lithium manganese cobalt nickel composite oxide powder was used. The lithium manganese cobalt nickel composite oxide was mixed as a positive electrode mixture by mixing 9 parts by weight of graphite powder and 2 parts by weight of carbon black as the conductive material 5 with respect to 85 parts by weight. An N-methyl-2-pyrrolidone (hereinafter abbreviated as NMP) solution (binder) in which polyvinylidene fluoride is dissolved so that polyvinylidene fluoride (hereinafter abbreviated as PVDF) is 4 parts by weight in this positive electrode mixture. Solution) and dispersed in NMP to form a slurry.

  Then, the slurry for positive electrode films produced above is apply | coated to the surface of the current collector foil 6 formed with aluminum using a die coater. Subsequently, the coated electrode film 9 was immersed in pure water for 5 seconds to solidify the binder 4 and then dried at 120 ° C. at a temperature rising rate of 3 ° / second in a warm air drying furnace to form a positive electrode film.

  Next, formation of the protective film 8 will be described. The protective film slurry was produced by the following method. Silica particles having an average particle diameter of 1 μm were used as the insulating particles 1. N-methyl-2-pyrrolidone (hereinafter abbreviated as NMP) in which polyvinylidene fluoride is dissolved so that 4 parts by weight of the polyvinylidene fluoride (hereinafter abbreviated as PVDF) is 75 parts by weight of the silica particles. The solution (binder solution) was added and dispersed in NMP to form a slurry.

  A protective film slurry is applied onto the positive electrode film produced above using a die coater. Subsequently, the coating protective film 8 was immersed in pure water for 5 seconds to solidify the binder 4, and then the protective film 8 was dried at 120 ° C. at a temperature rising rate of 3 ° / second in a warm air drying furnace.

As shown in FIG. 4, the binder distribution of the produced protective film 8 is such that the binder concentration on the electrode film 9 side is increased compared to the vicinity of the surface (opposite side of the electrode film 9), and the electrode film 9 of the protective film 8. The binder concentration on the side of the protective film 8 is 4.2%, the binder concentration on the surface side of the protective film 8 (the side opposite to the electrode film 9) is 3.5%, and the binder concentration on the electrode film 9 side of the protective film 8 is The binder concentration on the surface side (opposite side of the electrode film 9) was 20% higher than the binder concentration on the surface side (opposite side of the electrode film 9). Further, the binder distribution of the electrode film 9 is such that the binder concentration on the side of the current collector foil 6 is increased compared to the vicinity of the surface (protective film 8 side), and the binder concentration on the side of the current collector foil 6 of the electrode film 9 is 4. The binder concentration on the surface side of the electrode film 9 (protective film 8 side) is 3.7%, and the binder concentration on the current collector foil 6 side of the electrode film 9 is on the surface side of the electrode film 9 (protective film 8). The binder concentration on the surface side of the electrode film 9 (protective film 8 side) was 19% higher than the binder concentration on the side).
(Comparative Example 1)
Here, the slurry for the positive electrode film of Example 1 is applied to the surface of the current collector foil 6 formed of aluminum using a die coater. Subsequently, the coated positive electrode film was dried at 120 ° C. in a warm air drying oven at a temperature increase rate of 3 ° C./second to form a positive electrode film. Thereafter, the protective layer slurry of Example 1 was applied onto the positive electrode film using a die coater, and the protective film 8 was dried at 120 ° C. at a temperature rising rate of 3 ° / second in the hot air drying path. .

As shown in FIG. 4, the binder distribution of the produced protective film 8 is such that the binder concentration on the electrode film 9 side decreases compared to the vicinity of the surface (opposite side to the electrode film 9), and the electrode film 9 of the protective film 8. The binder concentration on the side of the protective film 8 is 3.6%, the binder concentration on the surface side of the protective film 8 (the side opposite to the electrode film 9) is 4.9%, and the binder concentration on the electrode film 9 side of the protective film 8 is the protective film 8. The binder concentration on the surface side (opposite side of the electrode film 9) was 27% lower than the binder concentration on the surface side (opposite side of the electrode film 9). Further, the binder distribution of the electrode film 9 is such that the binder concentration on the side of the current collector foil 6 is reduced as compared with the vicinity of the surface (protective film 8 side), and the binder concentration on the side of the current collector foil 6 of the electrode film 9 is 2. The binder concentration on the surface side of the electrode film 9 (protective film 8 side) is 5.3%, and the binder concentration on the current collector foil 6 side of the electrode film 9 is on the surface side of the electrode film 9 (protective film 8 side) ) Was 47% less than the binder concentration on the surface side (protective film 8 side).
(Effect of Example 1)
When the drying is performed after the binder 4 is solidified as in Example 1, the electrode of the protective film 8 is compared with the case of performing the hot air drying without solidifying the binder 4 as in Comparative Example 1. Since a large amount of the binder 4 is present on the film 9 side, the adhesion between the protective film 8 and the electrode film 9 is increased, and the protective film 8 and the electrode film 9 can be prevented from being peeled off. In addition, since the binder 4 is present on the side of the current collector foil 6 of the electrode film 9, the adhesion between the electrode film 9 and the current collector foil 6 is increased, and peeling between the electrode film 9 and the current collector foil 6 is prevented. It becomes possible. Thereby, since peeling from the electrode film 9 can be prevented without increasing the binder concentration, it is possible to suppress a decrease in battery capacity.

  Further, the difference between the binder concentration on the electrode film 9 side of the protective film 8 and the binder concentration on the surface side (opposite side of the electrode film 9) is set to 50% or less of the binder concentration on the surface side (opposite side of the electrode film 9). be able to. Furthermore, the difference between the binder concentration on the current collector foil 6 side of the electrode film 9 and the binder concentration on the protective film 8 side can be 50% or less of the binder concentration on the protective film 8 side. Thereby, since the part where a binder density | concentration is extremely low does not arise, particle | grain sliding can be prevented. Further, since the portion having an extremely high binder concentration is not generated, the pores on the electrode film 9 side are not blocked by the binder 4 and the battery capacity can be prevented from being reduced.

  Here, the case where the positive electrode film and the protective film 8 of the electrode film 9 are formed together will be described.

  The slurry for the positive electrode film produced in the same manner as in Example 1 is applied to the surface of the current collector foil 6 made of aluminum using a die coater. Then, the protective film slurry produced similarly to Example 1 is apply | coated using a die-coater on an application | coating positive electrode film | membrane. Thereafter, the coated positive electrode film and the protective film 8 are immersed in pure water for 5 seconds to solidify the binder 4 and then dried at 120 ° C. in a warm air drying furnace at a temperature rising rate of 3 ° C./second. A positive electrode film in which was formed was produced. Here, the method of applying the protective film slurry after first applying the positive electrode film slurry has been described. However, for example, multilayer coating in which the positive electrode slurry and the protective film slurry are simultaneously applied can also be used.

As shown in FIG. 5, the binder distribution of the produced protective film 8 is such that the binder concentration on the electrode film 9 side is increased compared to the vicinity of the surface (opposite side to the electrode film 9). The binder concentration on the side of the protective film 8 is 3.8%, the binder concentration on the surface side of the protective film 8 (the side opposite to the electrode film 9) is 3.3%, and the binder concentration on the electrode film 9 side of the protective film 8 is the protective film 8. The binder concentration on the surface side (opposite side of the electrode film 9) was 15% higher than the amount of field inter on the surface side (opposite side of the electrode film 9). The binder distribution of the electrode film 9 is such that the binder concentration on the current collector foil 6 side is increased compared to the vicinity of the surface (protective film 8 side), and the binder concentration on the current collector foil 6 side of the electrode film 9 is 3. The binder concentration on the surface side of the electrode film 9 (protective film 8 side) is 4.4%, and the binder concentration on the current collector foil 6 side of the electrode film 9 is on the surface side of the electrode film 9 (protective film 8 side) ) And 13% higher than the binder concentration.
(Comparative Example 2)
Here, the slurry for the positive electrode film of Example 2 is applied to the surface of the current collector foil 6 formed of aluminum using a die coater. Subsequently, the protective film slurry of Example 2 is coated on the coated positive electrode film using a die coater. The coated positive electrode film and the protective film 8 were dried at 120 ° C. in a warm air drying furnace at a temperature rising rate of 3 ° C./second to produce an electrode film 9 on which the protective film 8 was formed.

  As shown in FIG. 5, the binder distribution of the produced protective film 8 is such that the binder concentration on the electrode film 9 side is reduced as compared with the vicinity of the surface (opposite side to the electrode film 9). The binder concentration on the side of the protective film 8 is 4.8%, the binder concentration on the surface side of the protective film 8 (the side opposite to the electrode film 9) is 5.6%, and the binder concentration on the electrode film 9 side of the protective film 8 is the protective film 8. 14% of the binder concentration on the surface side (opposite side of the electrode film 9) was smaller than the amount of field inter on the surface side (opposite side of the electrode film 9). Further, the binder distribution of the electrode film 9 is such that the binder concentration on the side of the current collector foil 6 is reduced as compared with the vicinity of the surface (protective film 8 side), and the binder concentration on the side of the current collector foil 6 of the electrode film 9 is 2. 3%, the surface side of the electrode film 9 (protective film 8 side) is 4.8%, and the binder concentration on the current collector foil 6 side of the electrode film 9 is the binder on the surface side of the electrode film 9 (protective film 8 side) 52% less than the concentration.

Furthermore, diffusion of the insulating particles 1 (silica particles) forming the protective film 8 toward the positive electrode film side was observed, and 10% or more of the insulating particles 1 (silica particles) were mixed in the positive electrode film. Further, diffusion of the positive electrode active material in the positive electrode film toward the protective film 8 side was also observed, and the thickness in which 10% or more of the positive electrode active material was mixed in the protective film 8 was 20 μm.
(Effect of Example 2)
When drying is performed after the binder 4 is solidified as in Example 2, compared to the case of performing hot air drying without solidifying the binder 4 as in Comparative Example 2, it is the same as in Example 1. The effect is obtained. Moreover, in Example 2, since mixing of particles due to solvent movement during drying as in Comparative Example 2 can be prevented, the thickness of the protective film 8 can be reduced. When the protective layer is thinned, the amount of the positive electrode active material can be increased, and the capacity of the electrode film 9 on which the protective film 8 is formed can be increased. Thereby, when the positive electrode film of Example 2 is used, it is possible to obtain a lithium ion secondary battery having excellent reliability such as short circuit resistance and a high capacity.

  Here, as in Example 2, the positive electrode film of the electrode film 9 and the protective film 8 are collectively formed, and the binder 4 in the protective film 8 is double the binder in the positive electrode film of the electrode film. State.

  Then, the slurry for positive electrode films produced similarly to Example 1, 2 is apply | coated to the surface of the current collector foil 6 formed with aluminum using a die coater. Then, the protective film slurry produced similarly to Example 1, 2 is apply | coated using a die-coater on an application | coating positive electrode film | membrane. Thereafter, the coated positive electrode film and the protective film 8 are immersed in pure water for 5 seconds to solidify the binder 4 and then dried at 120 ° C. in a warm air drying furnace at a temperature rising rate of 3 ° C./second. A positive electrode film in which was formed was produced.

As shown in FIG. 6, the binder distribution of the produced protective film 8 is such that the binder concentration on the electrode film 9 side is increased compared to the vicinity of the surface (opposite side to the electrode film 9). The binder concentration on the side of the protective film 8 is 8.4%, the binder concentration on the surface side of the protective film 8 (the side opposite to the electrode film 9) is 7.8%, and the binder concentration on the electrode film 9 side of the protective film 8 is the protective film 8. The binder concentration on the surface side (the side opposite to the electrode film 9) was 7.7% higher than the binder concentration on the surface side (the side opposite to the electrode film 9). The binder distribution of the electrode film 9 is such that the binder concentration on the side of the current collector foil 8 is increased compared with the vicinity of the surface (protective film 8 side), and the binder concentration on the side of the current collector foil of the electrode film 9 is 4.4. %, The binder concentration on the surface side of the electrode film 9 (protective film 8 side) is 4.0%, and the binder concentration on the current collector foil 6 side of the electrode film 9 is the surface side of the electrode film 9 (protective film 8 side) 10% higher than the binder concentration.
(Comparative Example 3)
Here, the slurry for the positive electrode film of Example 3 is applied to the surface of the current collector foil 6 formed of aluminum using a die coater. Subsequently, the protective film slurry of Example 3 is coated on the coated positive electrode film using a die coater. The coated positive electrode film and the protective film 8 were dried at 120 ° C. in a warm air drying furnace at a temperature rising rate of 3 ° C./second to produce an electrode film 9 on which the protective film 8 was formed. Thereafter, the protective film slurry of Example 3 was applied onto the positive electrode film using a die coater, and the protective film 8 was dried at 120 ° C. at a temperature rising rate of 3 ° / second in the hot air drying path. In the dried electrode film 9, many cracks and peeling were observed in the protective film 8. As shown in FIG. 6, the binder distribution of the produced protective film 8 is such that the binder concentration on the electrode film 9 side is reduced compared to the vicinity of the surface (opposite side of the electrode film 9), and the electrode film 9 of the protective film 8. The binder concentration on the side of the protective film 8 is 7.1%, the binder concentration on the surface side of the protective film 8 (opposite to the electrode film 9) is 9.0%, and the binder concentration on the electrode film 9 side of the protective film 8 is the protective film 8. The binder concentration on the surface side (opposite side of the electrode film 9) was 21% lower than the binder concentration on the surface side (opposite side of the electrode film 9). Further, the binder distribution of the electrode film 9 is such that the binder concentration on the side of the current collector foil 8 is reduced compared with the vicinity of the surface (protective film 8 side), and the binder concentration on the side of the current collector foil 8 of the electrode film 9 is 2. The binder concentration on the surface side of the electrode film 9 (protective film 8 side) is 6.5%, and the binder concentration on the current collector foil 6 side of the electrode film 9 is on the surface side of the electrode film 9 (protective film 8 side) ) And the binder concentration was 57% less.
(Effect of Example 3)
In the case where drying is performed after the binder 4 is solidified as in Example 3, compared to the case where the hot air drying is performed without solidifying the binder 4 in Comparative Example 3, the same as in Examples 1 and 2. The effect is obtained. Further, in Example 3, since the stress due to the difference in drying shrinkage due to the difference in binder concentration between the protective film 8 and the electrode film 9 can be relaxed, it is possible to prevent the peeling from the electrode film 9.

  In addition, this invention is not limited to above-described embodiment, Various modifications are included. For example, the above-described embodiment has been described in detail for easy understanding of the present invention, and is not necessarily limited to one having all the configurations described.

DESCRIPTION OF SYMBOLS 1 Insulating particle 2 Solvent 3 Active material 4 Binder 5 Conductive material 6 Current collector foil 7 Evaporated solvent 8 Protective film 9 Electrode film 10 Binder concentrated layer 11 Solidified liquid

Claims (11)

An electrode film including a binder formed on the surface of the current collector foil and bonded to the current collector foil;
A protective film that is formed on the surface of the electrode film and includes a binder that adheres to the electrode film and insulating particles;
The lithium ion secondary battery, wherein the concentration of the binder on the electrode film side in the protective film is higher than the concentration of the binder on the opposite side to the electrode film.   The difference between the concentration of the binder on the electrode film side in the protective film and the concentration of the binder on the opposite side to the electrode film is 50% or less of the concentration of the binder on the opposite side to the electrode film. Item 2. A lithium ion secondary battery according to Item 1.   2. The lithium ion secondary battery according to claim 1, wherein the concentration of the binder on the current collector foil side in the electrode film is higher than the concentration of the binder on the protective film side.   The difference between the binder concentration on the current collector foil side and the binder concentration on the protective film side in the electrode film is 50% or less of the binder concentration on the protective film side. The lithium ion secondary battery as described.   The lithium ion secondary battery according to claim 1, wherein at least one component of the binder in the electrode film and the binder in the protective film is the same.   The lithium secondary battery according to claim 1, wherein the binder in the protective film is a polyvinylidene fluoride polymer, a rubber polymer, or a mixture thereof.   The lithium ion secondary battery according to claim 1, wherein the binder in the protective film includes a component that solidifies the protective film. A first step of applying a liquid protective film slurry containing a binder that adheres to the electrode film on the electrode film formed on the surface of the current collector foil;
A second step of bringing a solvent containing a solidified liquid into contact with the protective film slurry, and solidifying the protective film slurry;
And a third step of removing the liquid component from the solidified protective film slurry and drying it. A method for producing a lithium ion secondary battery, comprising:   The first solvent, which is a liquid component contained in the protective film slurry, is an aprotic polar solvent typified by N-methylpyrrolidone, dimethyl sulfoxide, propylene carbonate, dimethylformamide, γ-butyrolactone, or a mixture thereof. The lithium secondary battery according to claim 8, wherein the second solvent containing the solidified liquid is a protic solvent typified by water, ethanol, isopropyl alcohol, acetic acid, or the like, or a mixture thereof. Manufacturing method. A first step of applying a liquid electrode film slurry to the surface of the current collector foil;
A second step of applying a liquid protective film slurry to the surface of the electrode assembly to which the electrode film slurry has been applied;
A third step of bringing the electrode film slurry and the protective film slurry into contact with the electrode film slurry and the protective film slurry, and solidifying the electrode film slurry and the protective film slurry;
And a fourth step of removing the liquid component from the electrode film slurry and the protective film slurry and drying the liquid film slurry, and a method for producing a lithium ion secondary battery.   The aprotic polar solvent represented by N-methylpyrrolidone, dimethyl sulfoxide, propylene carbonate, dimethylformamide, γ-butyrolactone or the like as the first solvent which is a liquid component contained in the electrode film slurry and the protective film slurry, or these The second solvent containing the solidified liquid is a protic solvent represented by water, ethanol, isopropyl alcohol, acetic acid, or the like, or a mixed liquid thereof. Method for producing a lithium ion secondary battery.

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